The transfer film method is an industrially important technique for transferring a thin layer of metal to a substrate. This technique is used when the situation is not practical to metalize substrates directly. The substrate can be a flexible or rigid article such as polymeric film or paperboard sheet. The resulting metal coated substrates have various commercial end use applications among which packaging material is one good example. The metal on the substrate can perform different functions. Providing an aesthetically pleasing decorative appearance is a leading purpose for placing metal on the substrate.
Typical transfer film methods initially deposit a coating of metal to be transferred onto a surface of a carrier film. The carrier film usually has a polymeric base layer having a release layer coated on one side. The carrier film is made in a multi-step process of producing the base layer then coating release material on the polymeric base layer. The release material can be applied continuously directly after formation of the base layer. Alternatively in a discontinuous technique, the base layer can be isolated as uncoated film, stored for a time, retrieved from storage, and then coated with release material at the same or different location from that of base layer formation. Usually in a separate process, a metal layer is put on the release layer of the carrier film for example by vapor or sputter metal deposition thus forming the metal transfer film. The individual metal transfer film fabrication steps may be carried out at different locations by different converters. A disadvantage is that many processing steps are required.
The metal transfer film can be handled in film fabrication operations, optionally stored, and ultimately used in later steps of the transfer method. The release layer composition has metal adhesive properties for temporarily holding the metal on the carrier film during metal transfer film handling and releasing the metal at time of transfer. To transfer the metal, a traditionally liquid and/or tacky retention adhesive is coated onto the receiving surface of the substrate and then the exposed metal side of the metal transfer film is contacted with that adhesive coated surface. The retention adhesive is usually activated by applying heat and/or pressure that causes the metal side of the metal transfer film to bond to the substrate. Lastly, the carrier film is peeled from the substrate. Due to the weaker metal adhesion of the release layer compared to the retention adhesive, the carrier film separates from the metal layer leaving a durably adhered metal surface on the substrate.
The primary function of the release layer is to provide appropriately minimal adhesion between the polymeric film surface and the metal layer. The adhesion of the metal to the carrier film surface should be strong enough to endure handling in manufacture, packaging, shipping and similar operations of the metal transfer film prior to transferring the metal layer. However, release layer adhesion should be sufficiently weak that the metal layer cleanly separates from the carrier film surface when contacted with the substrate.
As a result of material handling during the metal transfer operation, the carrier film is generally not suitable for re-use directly following removal from the transferred metal. The spent carrier film might be recovered and regenerated by melting and reforming the polymer as film or raw material for some other product. However, the release material normally is a release agent dispersed in a polymeric binder that is different from the base layer polymer. The presence of release layer material in a melt blend of recovered carrier film adversely affects quality of the blend and thereby limits the potential uses of the blended product. There is a need for a metal transfer film having the transfer film consist of a single polymer composition and release agent such that polymer of a carrier film recovered from a metal transfer film can be recycled without contamination by different polymers present in the release layer.
Accordingly a method is needed for metal transfer from a polymeric carrier film that does not have a release layer with a foreign polymeric component. It is thus desired to have a complete carrier film for metal transfer in which the polymeric component is a single composition. This feature would enable recycling of the used carrier film after the metal has been transferred. It is especially desired to have a polyester-based carrier film and transfer film that is free of a release layer having polymer other than polyester.
To accomplish metal transfer as noted above, the liquid and/or tacky retention adhesive is deposited onto the surface of the substrate that is then brought into contact with the metal layer comparatively weakly adhered to the carrier film. After activation the cured retention adhesive durably and strongly bonds the transferred metal layer to the substrate. The carrier film then can be removed to leave the metal layer permanently affixed to the substrate. This technique calls for the fabricator of the finished metalized article to coat the retention adhesive on the substrate continuously and shortly before contact by the metal layer. This demands providing an inventory of adhesive, applying the adhesive, maintaining coating equipment, and disposing of any waste from the coating unit operations. It would be an improvement providing simplicity, convenience and increased productivity if the fabricator could use metal transfer film with a dry retention adhesive pre-coated on the metal surface.
Such a method has not been available heretofore because the retention adhesive is usually in liquid form with adhesively active components dissolved in a suitable solvent. Although organic solvents can be used, water is preferred because of the greater safety compared to processing organic solutions, and removing and handling recovered volatile organic solvent. Removing the solvent bonds the metal layer to the substrate. Solution form of the retention adhesive prohibits coating, packaging, storing and delivering the metal transfer film to the fabricator in advance of the metal transfer process step. A metal transfer film pre-coated on the metal layer with a dry, ready to activate, retention adhesive is needed.
Packaging for food and other odor generating substances is a common use for metalized articles produced using the metal transfer film technique. The contents of such packaging, especially foods, are vulnerable to spoiling from extended exposure to moisture and/or oxygen from air. Package material suppliers find it important to reduce the outward migration of odorous compounds from aromatic substances. It is also desirable to provide packaging that effectively inhibits the inward migration of ambient odor causing agents, moisture and oxygen from the environment for the purpose of extending the freshness of packaged goods. Each layer of a product metalized packaging material including a substrate polymeric film, the metal layer and the intermediate retention adhesive, of a packaging film, for example, can contribute to the barrier inhibiting transfer of odorous substances, water vapor and/or oxygen.
In the metal transfer film art various types of retention adhesives are available, for example, polyurethane, acrylic polymer, polyvinyl alcohol and ethylene vinyl alcohol copolymer and the like. However, they have metal adhesion and barrier properties of different effectiveness. It is very much desired to provide a retention adhesive that has excellent bond strength between metal and substrate and also has superior resistance to transmission of oxygen and moisture.